[ms-coff] Add a test for proper handling of full Windows path names in the .drectve section

Summary: This test ensures that we can correctly specify a full Windows path to the clang ASAN runtime libraries.  This is in preparation to fix PR20246.

Reviewers: rafael

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D4427
[SDAG] At the suggestion of Hal, switch to an output parameter that
tracks which elements of the build vector are in fact undef.

This should make actually inpsecting them (likely in my next patch)
reasonably pretty. Also makes the output parameter optional as it is
clear now that *most* users are happy with undefs in their splats.